The purpose of this study is to investigate functions of cell types within neoplastic and normal mammary glands (from humans and experimental animals). Areas to be investigated include the following: (1) the hormonal regulation of an endogenous peroxidase enzyme, (2) the cell type(s) responsible for iodide uptake, and (3) the cellular location of hormone binding sites. Recently it was discovered that estrogen induces peroxidase activity in certain mammary tumors that are hormone-dependent. This enzyme can be demonstrated cytochemically and was not found in hormone independent tumors, indicating its presence may distinguish mammary cancers that are hormone-dependent from those that are not. A peroxidase is also induced in the mammary gland during pregnancy, but the exact time of its appearance, and the hormones responsible for its induction have not been established. Ultrastructural cytochemistry will be used to detect endogenous peroxidase in the following mammary tissues: (1) in human female breast tissue (both neoplastic and "normal") obtained at surgery and immediate autopsy, (2) in mammary tumors arising spontaneously in mice and induced chemically by dimethylbenz(a) anthracene (DMBA) in rats, (3) in organ-cultured neoplastic and mid-pregnant mammary glands using defined media to identify the hormone(s) responsible for its induction, and (4) in the mammary gland in vivo during pregnancy. Correlative studies of the ability of the neoplastic and normal glands to concentrate radioiodide will be carried out using electron microscope autoradiography. This technique should identify the cell type(s) that concentrates iodide (i.e. the alveolar cell and/or the duct cell). Many functions of mammary gland, including differentiation, milk production and tumor growth are regulated by hormones, but the location (within this tissue) of binding sites for these hormones is unknown. An autoradiographic technique designed to localize diffusible substances, such as hormones, will be used to identify hormone binding sites within sections of normal and neoplastic mammary gland.